VMS Help SET, FILE, /RU_JOURNAL, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The SET FILE/RU_JOURNAL command marks an RMS file for recovery
    unit journaling. To use recovery unit journaling for a data file,
    a data file must be marked for recovery unit journaling with the
    SET FILE/RU_JOURNAL command, and transactions must be defined
    in an application program using DECdtm transaction services. You
    can also use this command to specify the default volume on which
    recovery unit journals will be created for this file.

    Use the SET FILE/NORU_JOURNAL command to unmark a file for
    recovery unit journaling. After you use the SET FILE/NORU_JOURNAL
    command for a file, modifications to that data file will no
    longer be written to a recovery unit journal.

    If you wish to delete a file that has been marked for recovery
    unit journaling, you must use the SET FILE/NORU_JOURNAL command
    before you can delete the file.

    There is no reason other than performance to keep recovery unit
    journals on a different volume from the file being journaled.
    Unlike after-image journaling, which protects against a system
    failure such as a head crash that causes a loss of data, recovery
    unit journaling ensures that a predefined set of operations are
    either done in their entirety, or not done at all. In the event
    of an abnormal termination of the application, such as a system
    crash or a Ctrl/Y, any incomplete transactions are automatically
    rolled back (undone). Because all recovery unit journals must
    be available before the data files can be rolled back, locating
    recovery unit journals on a volume where availability might be
    low could reduce the availability of the data files that use
    those recovery unit journals.

    Specifying a location for recovery unit journals for a file
    does not guarantee that the recovery unit journals will always
    be located on the named device or volume. For any active
    transaction, there is always only one recovery unit journal for
    local files. Thus, if many files are involved in a transaction, a
    single recovery unit journal is used, even if different locations
    for the journals had been specified (for individual files) with
    different SET FILE/RU_JOURNAL commands.

    Remote files are an exception to this rule. Each remote file
    associated with a transaction has its own recovery unit and
    recovery unit journal. The recovery unit journal resides on the
    remote system. The volume is chosen in the same way as for local
    files. Remote files have no effect in determining where the local
    recovery unit journal resides.

    A journal is not deleted when the transaction has been completed.
    Recovery unit journals are automatically deleted only when
    all of the files involved in the transaction are closed and
    the application exits. RMS journaling automatically creates a
    recovery unit journal at run time, whenever the first record
    stream associates with a transaction. All record streams in
    the process associated with the same transaction share a single
    recovery unit journal. Once a recovery unit journal is created,
    it can be reused for another transaction by the process that
    created it. A recovery unit journal is created only when there is
    no available recovery unit journal opened by the process for the
    current transaction.